Static Electricity and the Human Body

The human body model for electrostatic discharge (ESD) is 330 pF and somewhere around 8 - 16 kV, depending on charging conditions. The equation for energy stored in a capacitor is

E = (1/2)*C*V^2,

with E = stored energy, Joules

C = capacitance, Farads, and

V = potential, Volts

Plugging in the high end (16 kV) and solving for E yields

E = 0.17 Joule

To give a feel for how much energy that is, one Joule = 0.24 calories,

so

E = 0.17 Joule = 0.04 calorie

It takes one calorie to raise one gram of water one degree centigrade. So you are going to have to generate an awful lot of ESD to get energy that way.

Alternative uses for ESD are upset and/or destruction of sensitive electronics, and really annoying the family cat (sorry Del).

The human body is also resistive, which restricts the rate at which the energy can be dissipated in the outside world. The value typically used in simulations is about 1500 Ohms, but it can be lower than that if you can persuade the victim to hold a metal rod before discharging himself.
Other than that minor addition, I entirely agree with emc_c - there's not enough energy there to make it worth considering, even as a gimmick.

I left out the current limiting resistor, because it only complicates the computation of available energy (the topic of the original post, now long forgotten). My computation therefore maximized the available energy, and it is still a pittance. However, the actual value for the current limiting resistor used in IEC 61000-4-2 is 330 Ohms. They use 150 pF for charge storage. So I had those two numbers mixed up in my original post; which means the miniscule amount of energy calculated in that post overestimated the available total energy by a factor of two.

But what is of more interest is to calculate the ESD event time constant from the 150 pF and 330 Ohm values. It is 50 nanoseconds, not the one microsecond posited in another response. What may be of (academic) interest to the original poster is the amount of power delivered by the ESD event.

All we need to do is one again calculate the available energy, this time using the right value of storage capacity, and then divide by 50 ns.

E = (1/2)*C*V^2 = 0.0192 Joule

P = E/t = 0.0192 J/50 ns = 384 kilowatts!!!

where,

P = power (Watts),

E = total energy (Joules), and

t = time (seconds)

Please refer to my original post in this thread for definitions and values.

Being pedantic: with such a fast discharge, wouldn't most of the energy be dissipated in the internal resistor? The maximum power would be with the load resistor equal to the internal one, but the maximum energy would be with a relatively large load resistance.

Anyone care to compare this with using a generator to damp the movement of the door/light-switch or whatever you move as you discharge yourself?

I disagree that your post was off-topic, because the original question was about the amount of energy, and that is proportional to the event duration. Take yourself back on-topic :-)

For the very reason you cite, the exponential tail, it is common practice to list a duration by the time constant. I assume I am writing to an engineering audience and don't explain all those little nuances unless I think it is necessary.

I am familiar with the fast high amplitude initial spike, but it is so fast that the energy contribution is negligible. As you say, the major threat from ESD (aside from possible damage from a direct hit) is the coupling from nearby event into a susceptible circuit. Checking that is the entire point of the specification I cited in my original post, IEC 61000-4-2.

But you err when you say you need to keep a sensitive PCB in a shielded not just dissipative bag to protect against magnetic coupling. The coupling can only cause upset to a functioning PCB, and it isn't functioning in a bag. The coupling from an ESD event is not enough to cause damage, so if the bag prevents direct hits, that's all it needs to do.

"what if", I used an electric motor but instead of using the copper windings I used some sort of static conducting material how much electricity would you guesstimate it would generate?

It sound to me like you will end up with a Van der Graaff Generator (that is the bit of science equipment, not the rock band of the same name).

A couple of points: Static electricity is generated by insulating materials of different type rubbing against each other, not "static conducting material" (See http://en.wikipedia.org/wiki/Triboelectric_effect if you'd like to learn more about this). It may be collected and distributed by conductors and these are the same conductors as for any other sort of electricity, the only difference is that the voltage is so high that even highly resistive surfaces are still significant conductors for static. If you were thinking of materials like the ESD mats on workbenches, static conductive floors or the black coloured static dissipative plastic bags used to keep electronic components in they are "normal" conductors, but have a high resistance (like 1 MegOhm).

What the Van der Graaff Generator gives you is very high voltage, not very high energy. It may make your hair stand on end but it will not heat the water for a cup of hot coffee.

If you can get your hands on one of those old neon bulbs like they used to use to light clock dials and such, you can make it light by putting it in between you and the doorknob.

Of course, my favorite use is here:

http://www.pei.org/Uploads/static.mov

Hello bhankiii:

I have not seen anything like that before, amazing. I once worked in a steel framed building, It was several RSJ's about 12"/ 30cm square which stood off the ground 5 ft/ 150cm. It was a single story building. It started off as new with the same nylon carpet throughout, a 'freebie' by the builder.

It was uncomfortable to be in as there was such a large static build up, and as anyone turned the lights on, which were fabricated directly to the RSJ's, a spark would jump from your finger to the switch. It was a long time ago but I seem to remember (unusual for me!) the ESD was between 10/12 Kv. Certainly woke me up in the morning! We got round it by holding a coat over the arm before switching the lights on. It was my job to figure out was the cause was, and I said it is the carpet. And all the carpet was replaced with woolen carpet.No problems after that.

Take care................

Apart from members of staff with allergic asthma, of course

Hello Guest,

This is weird. I have had allergic eczema since I was 8 year old. Perhaps that is why I had so many shocks? Of course it is the dusty skin against my clothes which makes my ESC so high.

Take care......................

I can only wear pure cotton* clothing next my skin, and it must not be washed in detergents that contain any enzymes (= biological). If they are used, I break out into some "interesting" eruptions. At one point they changed the formulation of Persil to include enzymes and did not tell anyone; the consultant dermatologist thought I'd got extreme acne (silly **** as I was over forty at the time).

Any of this sound familiar?

*No wool, no synthetics, and not even treated "non-iron" cotton. And I have to have any parts of spectacles that touch my skin plated with an inert metal; that's the way you may recognise me if you meet me.

Hello Guest.

I echo pretty much all you say!

You could have taken the acne remark as a compliment, by saying you looked no more than 19?

I am walking round with a sweet wrapper which 'flew' to me a few moments ago. Static again.

Take care OK?

Hello all. While I have taken basic calculus, I am not nearly as mathematically inclined as many of you are. I stumbled upon your forum because of a search for information on my own situation, of which you may find interesting. I can light a 23w CFL bulb - with my bare hand. It does not fully power, but is highly obvious in a dark room, that the static generated by my body is enough to excite the gas in "trickles" (a very unscientific term, to be sure.) It is reminiscent of the static globes sold at stores as a gimmick toy. I am not registered, so if anyone wishes to reach me, please do so to my email at karmana1@hotmail.com.

As a side note, please turn off your cap lock key. All caps are considered in these forums to be shouting and rude. Thank you.

From what I recall it is generally reckoned that if you can feel a static discharge it is at least 3000 Volts. As was mentioned above is it not uncommon to get 8000V (the standard air discharge test for compliance with the European EMC directive and CE marking) and many more exacting test standards ask for 15000V or 16000V. Some Canadian standards ask for assessment to 30000V.

The "Electrostatic Discharge" (ESD) occurs in about 1 µs (microsecond). That's probably just as well - any longer and it might kill us.

Another interesting aspect is that the figure for the nominal capacitance of the human body given above assumes that you are not near a metal surface. When you are sitting in a car the figure is higher. Now it is not uncommon to generate static on your body when sitting in a car (especially if the seat cover and your clothes are man-made fabrics). If you step out of the car without first touching the metal of the car you decrease you capacitance without providing any way for the charge to get away. Q=CV (charge = capacitance times voltage). If you decrease the capacitance, but the charge remains the same the voltage must go up. Having got out of the car if you now touch the metalwork - ouch. Note this is the usual mechanism to produce a shock when you get out of a car, not the oft cited claim that the car body has generated static as you drive along. If you want to avoid the shock hold the metal part of you car key between your fingers and press the tip of the key against the metal of the car before you get out. This will discharge the static and it is unlikely you will feel anything.

I started to write a "tongue in cheek" suggestion that you connect a 1 MOhm resistor to the +12V of your car then press the end of the key against this so the discharge would charge up your car battery (a totally insignificant amount). But I really do not suggest you do this - you just might blow up the electronics in your car.

The way to use such a shock is, in my experience to get me to prevent another. As to how much, a lot. I put my hand out to the tip tank of a Learjet that had been flying around in Volcano weather, and got a great shock to my palm that made my elbow hurt. I was probably on my way to ground it and then fuel it, but those tip tanks are so shaped that it is common for a guy to reach out for a feel. I one time pissed on an electric fence, and the lesson is about the same. About the only use of static electricity I know of is a Tesla transmission experiment, or getting balloons to stick to cats. Funny Tesla invented AC.